The star map that BB-8 carries shows the system where Luke is at. IIRC, C-3PO says the system is "not on any of our charts". But the map shows at least a couple of dozen systems, and later we see it plugged into the whole galaxy and it's something like 10% of the volume of the galaxy -- a galaxy that has been explored and colonized for thousands of years.

How is it plausible that astrocartographers could not have figured out where this system was using only the smaller map held by BB-8, using just the positions of the systems shown (as well as the planets)? Wouldn't this be like being unable to identify Greenland or something? Is this just sloppy pop space fantasy writing or is there some way to make sense of it? Maybe the map was intended to be obscure somehow (like a treasure map)?

The reason mystery maps make sense in fantasy and old naval stories is because the world is supposed to be relatively unexplored. In an ancient galactic civilization this doesn't work so well.

It is unfortunate that when hundreds of millions are spent on a science fiction movie, a scientist can't be hired for a relative pittance to fill in the scientific holes. $50,000 would have gone a long way into applying some common scientific knowledge.
– Tony EnnisDec 30 '15 at 20:15

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They should have said that the map used an encryption that required multiple keys to decrypt and what BB8 carries is one if the required keys. There are algorithms already known to achieve this and would have made perfect sense.
– BakuriuDec 31 '15 at 16:24

@SS-3 except that it showed the endpoint of the line which, if the surroundings could be identified, should be enough. (Except maybe hyper lanes changes that puzzle a bit)
– zipquincyDec 31 '15 at 21:08

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@Martín-BlasPérezPinilla Remember that was a Warner Brother's foible - the original script had humans being used as a giant neural network-style computer, but they didn't think Average Joe would understand that.
– VogieDec 31 '15 at 21:37

9 Answers
9

Dutifully, the droid rolled into a suitable position. A lens brightened, and abruptly the lounge was all but filled with an enormously detailed and complex star map. Nebulae, solo stars, translucent splashes of concentrated dark matter, and entire solar systems were displayed before them. Even Chewbacca sat up to have a better look. Finn was impressed and Rey in awe — but Han found himself frowning.
Moving forward and into the three-dimensional representation, he tracked system positions and locator stars. One finger traced the outlines of a particularly bright and well-known nebular cluster. Like everything else in the map, it was brilliantly depicted.
It was also only half there.
He turned to the others. “This is accurate, but it’s not complete. It’s just a piece. I can tell from the location of the breaks and from what’s only partially shown.”

So... apparently, they can tell where in general this is (Han identifies a well known nebular cluster), but the specific pieces of the map are missing enough that it's impossible to get Luke's location (or the path to it) specifically, even knowing the neighbourhood.

I think this particular part of the novelization directly contradicts the movie, as well as common sense. If Han can identify a nebular cluster, then ipso facto he knows what portion of the galaxy the map fragment depicts. The whole "it's only a piece" conceit works if and only if they can't place the piece in the whole - like getting a fragment of a street atlas labeled "Walnut Ave." and "Main St.", because there are streets by those names in every largish city, and they often even intersect.
– MarthaDec 29 '15 at 17:10

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@DVK The map fragments the Resistance and New Order had showed the start of the path, and the final destination -- they were just missing the middle part. If you know the destination, you can just go directly there. Of course, this would have ended the film in 30 minutes... Not to mention, the New Order claimed they got it from the Empire's Archives, even though Luke wasn't hiding until after the Empire was destroyed. When you examine the movie, little of it makes any logical sense. Similar to how nobody knew how to destroy Death Star 3.0 until someone said "there must be a way to blow it up".
– SnakeDocDec 29 '15 at 17:19

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@SnakeDoc - that's definitely NOT the case in EU, some places are only accessible through a single, very well defined path. Plus you have "Hyperlanes". I agree movie is full of holes :)
– DVK-on-Ahch-ToDec 29 '15 at 17:22

The Essential Atlas is now classified as "Legends" rather than the new Disney canon, but as you can see from the Unknown Regions article on wookieepedia, such a region has been established to exist in the new Disney canon too, at least in times prior to The Force Awakens. According to the 'Appearances' section of the article it was specifically named as the "Unknown Regions" in the novel Tarkin. The article quotes a character named Berch Teller in Tarkin as saying:

"These aren't civilian targets. They're Imperial installations staffed by people who have bought into the Emperor's sick vision of the future—for you, your queen, me, and everyone between here and the Unknown Regions."

Also, the planet Ilum was established to lie in the Unknown Regions by the book Ultimate Star Wars which is part of the Disney Canon, and this planet was shown in the Clone Wars episodes "The Gathering" and "Test of Strength". Incidentally, the fact that the Jedi knew how to get to Ilum shows that the Unknown Regions were not wholly unknown, consistent with the point in DVK's answer that Han recognized a particular nebula on the map fragment.

An exclusive Lucasfilm star map reveals the name and location of three new planets in the Star Wars universe. ... Also out there, lurking in the galaxy’s darker recesses, is the Starkiller Base, home of the First Order. The map pinpoints its origin point in the Unknown Regions. Expect it to move somewhere more happening.

A low-res version of this new map has been posted on this page, you can see it shows the "Starkiller Base Origin Point" in the Unknown Regions on the left:

As for how it could be possible to have such a large region unmapped when the Republic existed for thousands of years and hyperspace travel seems to allow the galaxy to be traversed very quickly, I don't know--the "Legends" section of the article refers to possibilities like "gravitational anomalies" and "hyperspatial gravitic ripples", but it does note that there were instances of people traveling into this region in Legends canon, and from the main "canon" section of the article it doesn't sound like any explanation has been given so far in Disney canon (plus Rey was apparently able to make a reasonably fast trip in to find Luke). This might be a good subject for another question!

It seems especially silly for the unknown regions to be on the side of the galaxy that Coruscant is on!
– curiousdanniiDec 30 '15 at 2:37

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@curiousdannii - As long as the reason for it not being mapped isn't addressed in canon, my own headcanon idea would be that you need to know the exact location of the hyperlanes traversing a given region in order to travel it much faster than light, if you haven't already mapped the hyperlanes you might be limited to light speed, and the galaxy is said to have a radius of 50,000 light-years (in the movies going into hyperspace was referred to as a 'jump to lightspeed', which we could imagine as the initial speed until the ship reached a hyperlane)
– HypnosiflDec 30 '15 at 20:55

@curiousdannii -- As DVK highlights in his answer (and in comments to his answer) -- Hyperspace ain't Normal Space and seeing a map of the Hyperspace Route laid out on a Normal Space holo isn't really open to the criticism so many think it is. -- There are plot points to criticize in this movie but the Hyperspace Holo Map is not one of them people.
– user23715Dec 31 '15 at 1:56

@curiousdannii : Being on the same side of the galaxy as Coruscant does not mean it's close to Coruscant. Assuming the galaxy to be roughly the size of ours, that half of the galaxy is 50,000 light years (or approximately 1280 Kessel runs) wide. Plenty of room for an unknown region or two, even if you don't bring hyperlanes into the equation.
– RayJan 1 '16 at 1:03

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... Good telescopes can spot a lot of them at a fair distance, but considering that the estimate for the number of stars in our galaxy comes from estimating the mass and working backwards, this isn't necessarily easy. Exploring a galaxy, even at hyperspeed, would likely require making multiple stops to set up observatories and search for stars that were previously unknown. And then sending out those billions of probes to actually map the planets. Even ignoring travel time, this is a significant undertaking.
– RayJan 1 '16 at 1:46

As a software engineer, you get kind of used to seeing ridiculously simplistic graphical depictions of hacking on TV and movies. What I eventually had to start telling myself is that this is a fictionalized symbolic reference to what the character would have been doing, not a literal depiction of it.

I say this because I found myself having to do the exact same thing in that scene. What you are seeing on screen is a nice jumbo simplified version of a piece fitting in a puzzle, not a scale representation of the actual piece and the actual puzzle.

A real galaxy has 100's of billions of stars in it. Even if we assume most of those aren't what they call "systems", that's still way more systems than were physically represented in that scene, and any representation of a single "system" would take up next to no space at all in a scale model of its galaxy.

It's possible the size was accurate, but the visual display only showed a select number of "important" systems (say, ones with planets that had life)--there may have been more in the computer file than what was shown on the display, as is often the case with representing data graphically.
– HypnosiflDec 30 '15 at 20:07

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@Hypnosifl - I suppose a lot depends on how you interpret C3PO's statement implying the map is only of a single system. But either way, its clear a lot of information is being left out in that holographic visual representation the droids throw up, which is essentially my point. The holograms aren't the entire maps, just an immensely dumbed-down symbolic representation of them.
– T.E.D.Dec 30 '15 at 20:12

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According to the transcript here, C3PO says "I regret to inform you that the map recovered from BB-8 is only partially complete. And even worse, it matches no charted system on record. We simply do not have enough information to locate Master Luke." I'd interpret "it matches no charted system" to be specifically referring to the system identified on the map as the one where Luke is located, not suggesting the map as a whole was only a map of one system, since it showed lots of other dots.
– HypnosiflDec 30 '15 at 20:39

@Hypnosifl - That is essentially what I remember it saying, yes.
– T.E.D.Dec 30 '15 at 21:07

There is no answer about this in the film (dunno about the novel), but there is a possibility that the even the joined map wasn't a standard map.

When you think about it, there are probably a lot more stars in a galaxy other than those depicted on the map, but the map only specified some relevant ones in that context, so comparing it with a map that has all the stars in the galaxy would be very difficult, especially if you didn't know beforehand what size the small map was.

They didn't know that it was of the 10% of the galaxy showing only some stars. For all they knew, it could have been the map of a small system of stars in a small part of the galaxy.

I guess one could brute-force search for that specific pattern, but it'd probably take a lot of time.

Here is a slightly different suggestion; it was a problem of travel times. As has been established in many other answers, the location is within the Unknown Regions. There is no reason to suppose that we cannot pinpoint exactly where in said Unknown Regions, using the identifiable clusters, however, we do not have enough information of this region to chart a hyperspace route there.

Say we wanted to get there sans hyper-drive, how long would it take? The entire Galaxy is 100,000 light years in diameter, and just going by eye we might guess that the Unknown Regions are about a third of this. Let's assume that we only have to travel half way into them, we need to cover approximately 17,000 light years. So even if we travel very close the the speed of light for the entire journey, it will take a long time. Although the traveller may experience time dilation (no idea if this happens in SW) Luke would not and so would be dead before anyone arrived to fetch them.

So we really need a hyper-drive for this. Here is an extract about said hyper-drive from the (now non-cannon, but still roughly accurate) west end games RPG rule-book;

To get technical, hyperspace is coterminous to normal space. That is
each point in real space is associated with a point in hyperspace, and
adjacent points in real space are adjacent in hyperspace. If you head
north in hyperspace you travel north in real space. Real object have
a "hyperspace shadow". That is there is a star or star-like object in
hyperspace at the "same" location as it occupies in real space. This
is a danger.

Collision with a hyperspace shadow can "instantly vaporise" an object in hyperspace. So that's why we need nav computers;

All but the smallest hyperdrive-capable starships are equipped with
nav computers. Nav computers hold a wealth of statistical and map data
charting the positions of stars, rouge planets, known asteroid fields
and other dangers.

Only well charted regions are available on nav computers;

Going to a star system that has never been visited before is very
dangerous. None of the obstacles along the route have been charted.
Only the crazy (or desperate) would attempt to travel straight to an
unexplored star system; usually, explorers travel a short
distance through hyperspace, drop into real space to take readings,
travel a short distance again, and so on. This is time consuming, but
reasonably safe.

While the map may have given enough detail to physically identify the region, and it even looked like it had a partial hyperspace route (that red line thing), a whole hyperspace route would be require to get to Luke. Only when paired with the rest of the map was a whole hyperspace route, or enough information to plot one, available.

That is correct - they're also giant files. Massive. So large, in fact, that the astromech droids that can hold and chart other routes can only store a set amount (The R2 series can only hold 10, the R7s can hold 15) in memory at a time.
– VogieDec 31 '15 at 21:49

Following the rise and fall of the Empire, the New Republic does not have access to the same archives as the Old Republic. The starcharts they have on hand are not as complete as they once were when the Jedi Temple Archives were still around. As the Empire was the Old Republic, they still held this information, and R2-D2 downloaded this information at some point when it was interfaced with an Empire vessel.

and later we see it plugged into the whole galaxy and it's something like 10% of the volume of the galaxy

Maybe that's the false assumption? I don't recall enough detail to know whether the large map was "the galaxy". Maybe there are tens of thousands of holes in maps, but BB-8 communicated with R2D2 about which map to show, therefore indicating which hole matched BB-8's map? "the galaxy" is very specific, but if there were numerous such maps with uncharted areas, then BB-8's map might be less useful until more details are provided.

Keep in mind just how much access the Resistance had to R2D2's maps for most of the movie.

I love that people are trying to defend this whole premise. It was bad writing. Period. They could have gone with the option that Luke went into an unknown region of the galaxy where star chart data as sketchy, but they didn't. They went with the notion that they had the exact stars locations of where he was but that they couldn't figure that out based on even limited star data the New Republic (or even nav computers on Tatooine) would have had. They wanted the scene where R2-D2 has the whole map and they can plug in the missing piece like a jigsaw puzzle. Very 10 year old friendly yeah but not very smart.

What I think is even more crazy is that we are meant to believe that Starkiller base can suck down an entire sun but then that it has to belch out the stellar matter to destroy a planet. Why not just hyperspeed travel (as it would need to be able to do to be used more than once) and then kill the star of the target star system? Job done. Oh right but that wouldn't have looked so cool. The problem with SW-TFA is the same problem as Star Trek 2 by JJ Abrams. Rushed scripts = crappy scripts. The sooner we realize that and make the studio stop feeds us crappy scripts that look all cool and we all fan-boy/girl over, the better.

Besides, the question of why Starkiller Base does not travel to the Hosnian system is easily answered. It’s the same reason countries developed ICBMs, instead of sticking with dropping nuclear weapons from planes. Would you rather fly into the New Republic’s home space, sit there for a while, and give them time to evacuate or blow you up while you gather energy? Or would you prefer to hit them from halfway across the galaxy, where there is little risk of retaliation?
– AdamantOct 10 '16 at 22:28

Yes except that without the "Hyperspace window" that the beam was firing through (which was in the original script btw) it would have taken that beam like thousands of years or even millions maybe for that stellar matter beam to reach them. So your premise is slightly flawed. Like the movie.
– Jim StarkweatherJan 25 '18 at 22:30

I'm sorry, I don't understand what you're saying. The hyperspace windows was there, so what's the issue? Starkiller base could fire without exposing itself (more or less).
– AdamantJan 25 '18 at 22:34

The question is inaccurate since the map from BB8 was of a single system. The Larger map you refer to is shown by R2D2.

BB8 had a map of a single system but was only "partially complete" (not showing the full route of luke skywalker - althought now i think it did, because Rey found Luke in the same episode) and was not matching any of the resistance's chartered systems on record.

R2D2 having been in low power mode woke up and revealed the other part of the map, including the whole galaxy, leaving only one system missing. Now you could argue that R2D2 had knowledge beyond the resistance and withheld it before going into low power mode because he knew that without exact information of lukes route it would take huge resources to find him. Also it is likely that a part of the puzzle was removed by luke skywalker into a memory device from R2D2 so nobody knew where luke was, and somehow the man in the beginning of the movie had that memory piece which was then given to Po and then BB8 and then...

The resistance may have not seen any map for luke skywalker since C3PO said "it is very doubtful that R2 would have the rest of the map in his back-up data". (after seeing BB8's map)

If they hadn't seen the map from R2D2 (before BB8's map) they wouldn't have any movie shown clue of luke skywalker up until BB8.

So basically disney hadn't shown, in the movie, the reaction of "R2D2's hiding of the map" in any of the characters to make the movie clearer for those who didnt realise earlier that R2D2 hadn't participated in finding luke.

Like C3PO could have said "why didnt you show us this earlier, you stupid piece of junk."

But by the looks of it R2D2 had some bad feelings about the whole experience of losing his master and so went into low power mode up until the other part of the map was found. (he justified waking up having the knowledge he could potentially aid the where abouts of luke's location accurately alongside BB8's map)